VIP and cyclic hematopoiesis: During CH attacks the trigeminal-autonomic reflex (an association between the trigeminal sensory system and the parasympathetic system—sphenopalatine and otic ganglia) is activated provoking vasodilation of cranial arteries by the release of vasodilatory molecules, including CGRP, vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) [20,104].